This invention relates to adhesive systems used in the manufacture of certain products, to the products which are formed using these adhesive systems, and to methods for producing the respective adhesive systems and the subject products. More specifically, this invention is directed to adhesive systems comprising aldehyde resins and polymeric isocyanates used in the manufacture of certain lignocelluosic products, to the lignocellulosic products which are formed using these aldehyde resin-polymeric isocyanate adhesive systems, and to methods for producing these aldehyde resin-polymeric isocyanate adhesive systems and lignocellulosic products.
At present PF resins still dominate the worldwide market for the production of exterior grade wood-based panels. However, PF resins present a problem in that a variety of formulations are employed due to the very different types and forms of wood used. Furthermore, PF resins have some serious limitations. For instance, PF resins are not able (i) to consistently bond wood panels of higher moisture content (8% to 12%), (ii) to form high quality, structural panels from medium density veneers and high density veneers (0.6 to 0.8 Kg/m3, and to accomplish all the above with lower adhesive spread levels. A resin of any nature or composition which is capable of overcoming all the defects of PF resins listed above, while still maintaining their excellent performance, would indeed be a great step forward in adhesives for exterior grade wood based panels.
Isocyanates (in particular MDI polymers) have been known in the wood industry for almost 25 years. These materials can produce excellent results when used as adhesives for particulate products but have been widely recognized as totally unsuitable for plywood. This is due to their low viscosity which makes these resins migrate from the glue-line into the veneers before and, more particularly, during plywood hot-pressing operations. This results in completely starving the glue-line of adhesive and leading to joints of very poor strength. A composite resin containing MDI which would be capable of utilizing the great strength capabilities of MDI, but which is capable of also bonding plywood, would become as successful in the field of plywood manufacture as MDI is successful today in bonding wood particle products.
In isocyanate and polyurethane chemistry, the belief was that in mixed resins containing MDI/phenol-formaldehyde, or MDI/melamine-formaldehyde and MID/urea-formaldehyde resins, the isocyanate group can react not only with water but also with the phenolic hydroxy groups of phenol of a given resin, and possibly with the amidic groups of urea and melamine resins. In reality, the latter reactions are slower than the reaction of MDI with water, and thus, such a system would largely lead to MDI deactivation, which is what has always been believed.
What has always been disregarded is that the isocyanate group (--N--C--O) can and does react extremely rapidly with the methylol group (hydroxybenzyl alcohol group) present in PF-MDI resols, and with the equally reactive methylol group present in MF-MDI and UF-MDI resins. The initial results indicate that this is indeed the case. Thus, the rate constants shown in Table 1 of U.S. Pat. No. 4,833,182 indicates that the reaction of the isocyanate group of MDI with methanol, and with the CH.sub.2 OH groups of PF-MDI, MF-MDI and UF-MDI resins, occurs readily.
U.S. Pat. No. 5,407,980 attempts to solve the problems associated with combining a polymeric isocyanate and an aldehyde resin through the use of a tannin accelerator to modify the above-described problem reactions associated with conventional isocyanate-aldehyde chemistry. In summary, the isocyanate reacts with the phenol-formaldehyde resin, with the tannin, with the water and, probably, with the paraformaldehyde. The phenol-formaldehyde resin reacts with the isocyanate, with a tannin accelerator and with the paraformaldehyde. The tannin reacts with the isocyanate, with the phenol-formaldehyde resin and with the paraformaldehyde.
In U.S. Pat. No. 3,919,017 ("US '017"), isocyanate-formaldeyde binder systems were used in the manufacture of products from lignocellulosic materials, particularly particleboard. The particleboard was fabricated by the conventional manner of application employed in the prior art, namely, by spraying the lignocellulosic particles with binder as they are tumbled or agitated in a mixer. It is a basic understanding in the prior art, as expressed in US '017 in column 3, at lines 58-60, that the binder system components are separately added or applied to the lignocellulosic particles in order to avoid premature reactions between them. When the separate addition method of US '017 is employed, the order of addition of the respective binder system components is considered to be unimportant. EXAMPLE 1 of US '017 describes a typical method of separate binder component spray addition or application to lignocellulosic materials.
In U.S. Pat. No. 4,209,433 ("US 433"), the approach advanced in US '017 is modified by specifically delineating the order of spray addition of the binder system components. To achieve the advantages of the method of the present invention, the isocyanate polymer is applied to the lignocellulosic particles prior to the application of the aldehyde resin. Thereafter, the combined adhesive is reacted by heating same in situ to obtain an improved thermosetting adhesive resin and the advantages produced thereby. In accordance with the method of US '433, the prior application of the isocyanate polymer creates enhanced adhesive characteristics as well as improved tolerance of the adhesive to the wood species and to the process variables. According to US '433, upon initial application, the isocyanate polymer reacts readily with hydroxyl groups or water on the surface of or among the wood particles to form strong molecular bonding. Subsequently, a cross-linking reaction between isocyanate groups and the phenolic resin occurs, upon the introduction of the phenolic resin, thereby reinforcing the functionalities of the phenolic adhesive. The initial rapid reaction of the isocyanate polymer gives the combined resin system its improved tolerance to the wood species and to the process variables.
The method of US '433, and the advantages produced thereby, are not dependent upon the particular method utilized in the fabrication of the particle board, except of course, the requirement that the isocyanate polymer be applied prior to the phenolic resin. In general, the method for fabricating the particleboard involves the application of the adhesive composition, isocyanate polymer first, to the wood chips with subsequent application of heat and pressure to form the board into its desired configuration. The isocyanate polymer and phenolic adhesives are separately applied to the wood chips by any conventional means such as spray coating, coating or other dispersion of the adhesive and wood. Unfortunately, this adhesive system and application method fails to provide a satisfactory result for use with lignocellulosic materials such as OSB due to the low resin content used.
All of the issued patents cited in this application are incorporated in their entirety by reference.